Stencil Foil Thickness: 100 vs 120 Micrometre Foils

Stencil foil thickness is the single largest lever on deposit volume, because the aperture area is fixed by the pad while the height is set by the foil. Move from a 120 micrometre foil to a 100 micrometre foil on the same apertures and the deposit volume drops by close to a sixth. That is why thickness is chosen early and changed reluctantly.

The choice is a compromise between the volume a joint needs and the area ratio the apertures can support. Thick foil holds more paste but releases it less cleanly on small openings, and thin foil releases well but starves joints that demand volume. This note sets out how to pick a thickness and how to qualify the change.

What Foil Thickness Controls

Thickness sets deposit height, and deposit height multiplied by aperture area gives the paste volume delivered to the pad. It also sets the aspect ratio of the aperture, which is the ratio of opening width to foil thickness. An aperture that is narrower than about 1.5 times the foil thickness becomes difficult to fill and difficult to release.

Thickness also decides how much the stencil deflects under the blade. A thin foil with good tension flexes slightly and gaskets well against the pad, while a thick foil holds its shape and relies on the solder mask for the seal. Both behaviours are acceptable, but they demand different print gap and pressure settings.

Area Ratio and the Practical Limit

The area ratio of an aperture is the pad contact area divided by the wall area, and it is the number that decides whether paste leaves the stencil. Below roughly 0.66 the deposit becomes unreliable, and below 0.5 it is not achievable in production. Area ratio falls when the aperture gets smaller or the foil gets thicker, so a thickness increase can push a marginal design over the limit.

This is the reason fine pitch designs are printed with thin foil and coarse designs with thick foil, and the reason a single stencil cannot serve both extremes without a step. The working figures for each paste class are listed in our notes on stencil aperture area ratio.

Thin Foil and the Paste Volume Trade-Off

A thinner foil is the standard answer when fine pitch apertures will not release, but it removes volume from every other aperture on the board. The joints that suffer first are the through hole parts and the large thermal pads, where the paste has to fill a barrel or cover a wide area. Starved joints of that kind pass inspection and fail in the field.

The correction is to add area rather than height. Opening the stencil beyond the pad on the coarse features, within the limits the mask window allows, recovers volume without touching the fine pitch openings. This is the trade that makes a single thickness workable, and it is calculated from the paste volume requirement of each joint class.

Step Stencils for Mixed Boards

Where area is not available, the foil itself can change height. A step stencil has a thin region for the fine pitch devices and a thick region for connectors, thermal pads and through hole filling. The transition is milled or etched into the foil, and the step is placed away from critical apertures so the blade does not fall off the edge of the raised area.

Steps of 20 to 40 micrometres are routine. Larger steps stress the blade and leave a residue line that has to be wiped, and they also need a stencil with a machined pocket rather than an etched one so that the transition is smooth. Section changes should be reviewed with the fabricator before the step stencil design is released.

How Thickness Affects Paste Release

Release happens at the aperture wall, and the wall area grows with thickness. More wall area means more friction and a longer distance for the paste to travel as the stencil lifts, which is why a thick foil holds paste back on small apertures. The foil finish, the wall taper and the coating applied by the stencil maker all act on the same interface.

A thin foil shortens that path and releases cleanly, but it also gaskets less material against the pad, so the seal depends on the print gap and on the flatness of the board. Where a thin foil is used, the stencil must sit flat and the rails must be clean, because paste will find any gap between foil and pad.

Choosing Stencil Foil Thickness: 100 or 120 Micrometres

Start from the smallest aperture on the board and calculate its area ratio at each candidate thickness. If a 120 micrometre foil leaves the finest aperture above 0.66, take the thicker option and gain volume everywhere. If it does not, the choice is a 100 micrometre foil with more open apertures, or a step.

The second consideration is the joint class that needs the most paste. A board with a fine pitch package and nothing else can run thin foil throughout. A board that also carries press fit connectors, shielding frames or pin in paste holes usually cannot, and the step is cheaper than a reflow defect that appears months later. Design guidance for the foil itself is published in the IPC stencil standards.

Foil Thickness and Stencil Tension

Thin foil requires higher tension to stay flat, and a frame that was adequate for 150 micrometre material may not hold a 100 micrometre foil without sag. A stencil that sags between the apertures loses its seal in the middle of the panel, and the symptom is paste bleed that varies with position. Measuring tension when the stencil is received is the only reliable check.

Thick foil behaves in the opposite way. It holds its own shape, sits on the high points of a worn stencil bed and prints correctly even when the printer is not perfectly set up. That tolerance for a poor setup is a benefit on a stable product and a source of hidden variation on a fine pitch one, which is why the same stencil specification should not be copied between product families without review.

Qualifying a Thickness Change on the Line

A thickness change alters volume on every pad, so it is a full process change rather than a tweak. Print a first article on the new foil, measure the deposit volume on the finest and the coarsest apertures, and compare with the previous data before the balance of the batch is released. Reflow the first article as well, because volume that looks acceptable on the printer can still leave a starved barrel.

Record the foil thickness, the tension and the print settings together, so that a future volume difference can be attributed. Where the change is made to solve a release problem, keep the old data in the same file, because the thickness may have to be reversed if the coarse joints are affected more than expected.

FAQ

Can a stencil be thinner than 80 micrometres? Yes, foils down to about 50 micrometres are available for very fine pitch work. They are fragile, they need a high tension frame and a flat stencil bed, and they remove so much volume that any joint needing paste in a barrel must be fed another way.

Does a thicker stencil always bridge more? Not by itself. Bridging comes from excess volume and poor release, and a thick foil supplies more of both. The same apertures on a thinner foil with adequate area ratio usually print without bridges, which is why the thickness is the first variable to question.

How often should stencil foil thickness be verified? At receipt and after any frame re-tensioning or repair. Measure the foil with a micrometer across several points, because rolling tolerance and local thinning near a step both change the volume delivered to the pad.